William Walker held on longer ITTL, long enough to throw his lot in with the Confederacy in the Civil War. The Union launched a pre-emptive naval expedition that overthrew him before that could happen, after which the Nicaraguans were pressured into filing for annexation, partly to give the US control over a good canal zone, and partly to secure control of the country's economy.

I like the map (wish I had a link to the thread on AH.com), but the sheer amount of rivers that aren't particularly major ones like the Mississippi, Ohio, or Colorado sort of takes away the focus for me.